There have been recent proposals, such as Japanese patent laid-open publication number 62(1987)-26757, for an inductively coupled plasma mass spectroscopic apparatus that utilizes ions to perform an element analysis in a solution.
Referring to FIG. 1, such inductively coupled plasma mass spectroscopic apparatus includes a sample introduction section 10, a gas control section 18, an ionization section 20, RF power supply 26, an interface section 30, ion lens section 40, a mass selection section 50, a mass filter drive circuit 56 and an ion detection section 60.
The sample introduction section 10 nublizes a solution containing sample components to be analyzed, i.e. a sample solution and introduces a sample solution into an ionization section 20. The sample introduction section 10 further includes a peristaltic pump 12 that continuously brings sample solution 8 into a spray chamber 16, and at the same time, pumps exhaust liquid out of the spray chamber, and a nebulizer 14 which nebulizes sample solution 8 that has been sampled by peristaltic pump 12, and spray chamber 16 which separates finer droplets from the nebulized droplets (neublized by nebulizer 14).
The gas control section 18 injects a carrier gas (e.g. Ar gas) into a nebulizer 14.
The ionization section 20 atomizes and ionizes elements in droplets containing the sample solution that is carried with a carrier gas from the sample introduction section 10. The ionization section 20 further includes a torch 22 and an induction coil 24 that is wound around the outside torch 22.
The RF power supply 26 supplies a high frequency power to induction coil 24 to generate a plasma.
The interface section 30 samples the ionized elements within ionization section 20 at an atmospheric pressure and introduces them to the ion lens section 40 under a high vacuum. Interface section 30 includes a sampling cone 32 that controls a direction of a kinetic energy of each ion which is generated by torch 22, and a skimmer cone 34 that passes a portion of the ions which have been passed through sampling cone 32.
The ion lens portion 40 converges the ions that pass through interface portion 30 and guides them to a mass spectrometer portion. Ion lens portion 40 includes an electrode 42 that extracts the ions from an orifice of the skimmer cone, an Einzel lens 44 that converges the ions and an electrode 46 which transports the ions into the mass selection section 50, all of which functions as electrostatic ion lenses.
The mass selection section 50 includes, for example, a quadrupole mass filter 52 comprising four electrode rods 54.
The mass filter drive circuit 56 drives quadrupole mass filter 52.
The ion detection section 60 counts the ions of the mass number (being measured) from mass selection portion 50, and includes, for example, a secondary electron multiplier tube 62.
In FIG. 1, a vacuum exhaust system 70 includes a rotary pump 72 that creates a vacuum within an interface chamber 38 formed by sampling cone 32 and skimmer cone 34; a turbo molecular pump 74 that creates a high vacuum within ion lens chamber 48 of ion lens section 40; a turbo molecular pump 76 that creates a high vacuum within analyzer chamber 58 in which quadrupole mass filter 52 and secondary electron multiplier tube 62 are enclosed; and a rotary pump 78 that creates a low vacuum for turbo molecular pumps 74 and 76. A system controller 80 controls portions of the apparatus such as sample introduction section 10, gas control section 18, RF power supply 26, mass filter drive circuit 56, detection section 60, and vacuum exhaust system 70. A workstation 82 provides instructions to system controller 80 and performs data collection and analysis of the spectroscopic data.
The inductively coupled plasma mass spectroscopic apparatus (as described above) allows a carrier gas to flow into torch 22 where a high frequency electric field is applied to induction coil 24 to create a plasma into which nebulized sample solution 8 is introduced, in order to ionize the elements within sample solution 8. The ions pass through interface section 30 which is comprised of sampling cone 32 and skimmer cone 34, and enter ion lens portion 40. Then, the quadrupole mass filter 52 selects the elements based on their mass and an electron multiplier 62 detects them. The lower limit of detection can be as low as sub ng/L (ppt) for most of elements, thereby an element analysis with very high sensitivity can be achieved.
However, a low efficiency of an ion transmission from ion lens section 40 to mass selection section 50 limits a detection capability of an inductively coupled plasma spectrometer. Such transmission efficiency is one of indexes showing a performance of the ion lens section including an electrostatic ion lens and/or an ion beam guide. It is defined as B/A, where A represents a number of ions which enter the ion lens section and B represents a number of ions which come out of the ion lens section and enter the mass spectrometer section. In particular, a multi-stage arrangement in which a plurality of lenses are arranged in series requires a complex ion lens system and a difficult and complicated alignment.
Another problem is a transmission of undesired particles to a detector, such as vacuum ultraviolet photons generated in a plasma, which cause continuous background to be appeared in a spectrum and thus, lowers S/N ratio or a detection capability of the detector.
Accordingly, a first object of the present invention is to enhance the ion transmission efficiency of the ion lens section including an ion beam guide and to facilitate an adjustment (alignment) of the apparatus.
A second object of the invention is to prevent the light, which repeatedly reflected (multiple reflection) off an inner surface of a multipole ion beam guide, from reaching the ion detector.
A third object of the invention is to provide an improved and easy maintenance for an ion beam guide.